1. Field of the Invention
This invention relates to a method of transmission of information or measuring data and device for using the method.
This method can advantageously be applied in telemetry devices by making it possible, for example, from a central set or central station to interrogate local stations that measure certain parameters such as, for example, consumption of fuel by a boiler, distribution of gasoline at service stations or depots, consumption of household fluids such as electricity, gas, water.
Other applications such as measurement of service or access to services or data bases can also be envisaged.
2. Discussion of Background
Such telemetry devices are known in the prior art and are classified in three categories. A first category of devices relates to devices whose central acts like a slave and is called by the local stations. These devices have the drawback that several local stations can decide at the same time to access the central and, in this case, a collision occurs.
In second category of device, the central operates as a master and decides on the calls to the local stations. In these devices, the central sends a call by the telephone lines to the local station and the latter, as a result of this call, sends the measuring results. These devices have the drawback that initiation of a telephone call to the set suffices to obtain transmission of the measurements, which can be annoying.
In this same category of devices where the central is master, an improvement has been made which consists in having a call sent by the central and having this call followed by an interrogation signal. This signal, depending on the devices, can be produced before or after the ringing signal. In case the interrogation signal is produced before the ringing signal, this interrogation signal enables the local station to tie up the subscriber's line before the ringing signal has been sent. This avoids disturbing the subscriber unnecessarily. In this type of device, the interrogation signals consist either of a direct current which is sent over the telephone line or of a voltage drop or increase in relation to the normal voltage of the line or, finally, of a given frequency. The latter type of device with an interrogation signal consisting of a given frequency can be illustrated by European patent application No. 0014774. Such devices, if they do offer an improvement in the security of the connection between the central and the local station, and consequently of the transmission, still have flaws. Actually, a cheat could detect on the telephone line the frequency signal sent to interrogate the local device and then make calls by having them followed or preceded by this frequency signal and thus cause transmission of data and possibly resetting of the counters or memories which allow storage of the measurements. Further, such devices require considerable modifications of the existing telephone installations.
On the other hand, it is known by the patent application FR 2 373 109 to use a precall procedure followed by a call on a telephone line at the end of which a remote control instrument is connected which will make it possible to trigger remotely the starting of a household device, such as, for example, a boiler. In this device, the precall allows presensitizing of the receiver while the second call constitutes an order execution signal. It is even possible to envisage remote control of several instruments by the same line by causing the number of precall rings to vary as a function of the device to be controlled. In the device of French patent application No. 2 373 109, the calls that follow the precall should comprise a number of rings equal to the number of precalls except for a constant factor K.
This type of remote control device with the precall procedure is intended to be used for a dwelling that is normally vacant since the remote control of an instrument is desired and consequently the number of rings and the ringing are not a bother to the subscriber who is absent.
Finally, anyone, who could call the line to which the remote control instrument is connected, could very well allow ringing for a number of times corresponding to the precall, hang up and, not obtaining any answer, call again after a time and allow ringing for a number of times corresponding to the call. The result of this fortuitous maneuver would be triggering of the remote control. Consequently, this type of instrument does not make it possible to distinguish the calls really corresponding to a remote control from chance calls for which an action should not be executed.
In French patent application No. 2 500 246, a device is provided in which detection of the ringing trains is performed by detectors that supply signals when a call signal is received on a line. This device therefore takes into account the appearance of the call signal and does not take into account the period or even the nature of this signal. This device has the drawback of being very sensitive to spurious signals. Further, the use of detectors that are triggered by the appearance of call signals to cause either the connection or suspension of the ringer causes this suspension to be incomplete and there will always be a slight ringing corresponding to the appearance of the variation of the call signal.
It has also been proposed in British patent application No. 2 097 223 to trigger the connection or disconnection in parallel by the detection of the interval existing between two call signals, this triggering occurring when said interval is greater than a visual interval. This arrangement has several drawbacks, in particular that of being very sensitive to the appearance of spurious signals or also to the appearance of a second call completely independent of the first call (call of another subscriber).